In controlling an electric motor (motor), it is necessary to detect a motor driving current with high accuracy. For this purpose, measurement accuracy of a current sensor is improved by properly detecting an electric current offset of the current sensor and correcting the detected offset to thereby learn a zero point of the current sensor (an output value of the current sensor when current=0) over time.
For example, there is a configuration, as disclosed in Japanese Patent Laying-Open No. 10-80172 (Patent Document 1), wherein, for a current sensor (current detector) for detecting driving current of an electric motor, the time interval between a time point when a detected current of the current sensor becomes maximum and a time point when the detected current becomes minimum is calculated successively and an electric current offset arising in the current sensor is detected based on a change in the time interval successively obtained between the maximum point and the minimum point.
With the configuration disclosed in the Patent Document 1, it is possible to properly detect the electric current offset arising in the current sensor to perform offset correction irrespective of a state of the electric motor.
There is an electric current offset value calculator of a semiconductor magnetic sensor generally used as a current sensor mounted on an electric car or the like as disclosed in Japanese Patent Laying-Open No. 2004-191301 (Patent Document 2).
According to the current sensor offset value calculator disclosed in the Patent Document 2, for a magnetic flux detection type ammeter (current sensor) provided for measuring the electric current input to and output from a secondary battery, a current value detected by the current sensor is calculated as an offset value when the polarity of the detected electric current is changed and the current sensor is determined to be in a non-energized state. Especially, in the current sensor value offset calculator disclosed in the Patent Document 2, turning off of an ignition switch is detected to determine the current sensor to be in the non-energized state.
In general, a motor driving device includes a power converter circuit such as an inverter for converting DC power from a secondary battery into AC power for driving an AC motor. In the power converter circuit, electric power semiconductor elements carry out switching operations at high frequencies, which generates electromagnetic wave noise. Therefore, not only the spatial and temporal change in the external magnetic field disclosed in the Patent Document 2 but also the electromagnetic wave associated with the switching operations of the electric power semiconductor elements may become noise for the current sensor. Especially, the electromagnetic wave noise may act on a control board mounted with an integrated circuit (IC) and the like for controlling the current sensor to reduce accuracy of the offset correction (zero point, adjustment).
A vehicle driving motor mounted on a vehicle such as a hybrid car, an electric car, or the like is subject to severe layout constraints from a viewpoint of maintaining spaces for occupants and storage in the vehicle. Moreover, the vehicle driving motor needs be driven by relatively large electric current and therefore the switching operations in the power converter circuit involve electromagnetic wave noise of relatively high intensity.
In a motor driving system including a plurality of motors and a plurality of motor driving devices disposed close to each other and represented by a casing where a plurality of vehicle driving motors are mounted on a hybrid car or the like, an error may be caused in offset correction (zero point adjustment) of a current sensor in each motor driving device by noise from the other motor driving devices. In this point, no problem is pointed out in the Patent Documents 1 and 2.
Especially, in the vehicle driving motor mounted on the hybrid car, the motor output need be controlled with high accuracy so as to obtain driving performance according to a requirement of a driver. In general, the motor driving control represented by output torque control involves feedback control of the motor driving current. Therefore, if the zero point adjustment of the current sensor is not carried out properly, an influence of a detection error of the motor driving current causes variation in the motor output (represented by torque pulsation) and an especially large detection error may lead to occurrence of vibration of the vehicle. Consequently, for such use, it is necessary to properly carry out the zero point adjustment of the current sensor to suppress the detection error of the motor driving current.